This invention relates to depositing platinum onto a zeolite carrier. More particularly this invention provides a process for loading and uniformly distributing platinum into a Type L Zeolite by ion exchange. Platinum loaded Zeolite-L thus prepared is especially useful as a catalyst, particularly in the dehydrocyclization of paraffins into aromatic compounds.
Zeolite-L, particularly the potassium exchanged form of Zeolite-L, is a non-acidic carrier and in order to obtain its unique catalytic properties after platinum loading, particularly its unique dehydrocyclization capabilities, it is important that the zeolite remain non-acidic, and that acidic sites not be generated due to the platinum loading procedures. In addition, in order to enhance the performance of the platinum loaded Zeolite-L catalyst, a uniform distribution of platinum throughout the Zeolite-L is important. Another important requirement includes carrying out the loading procedure without leaving excess salt or ions in the zeolite pores which could lead to undesirable obstruction and a consequent reduction of catalytic activity. In addition, since it is commercially desirable to load a bound form of the Zeolite-L in the form of a pellet, the loading should be carried out in a manner so as not to cause excessive attrition of the pellets when processed in large quantities. Further, the loading must be carried out in a manner which is suitable for commercial scale operations.
The two basic methods of loading platinum into a zeolite carrier using an aqueous platinum solution are the impregnation and ion exchange techniques. The impregnation technique of loading platinum into a zeolite carrier generally involves loading with an amount of cationic platinum solution of a volume only sufficient to fill the total pore volume of the carrier to incipient wetness (saturation). In contrast, the ion-exchange technique involves loading platinum into a zeolite carrier with an amount of cationic platinum solution in excess of that needed to fill the total pore volume of the carrier to incipient wetness. The excess solution is stirred with or circulated through the bed of zeolite particles. In both cases there is a rapid decrease in the concentration of platinum ions to a minimum and an equivalent increase of the non-platinum cations in solution due to the ability of the zeolite to incorporate other cations via ion-exchange with the non-framework metal ions of the zeolite. Completion of the catalyst preparation consists of drying and calcination of the solids. In the case of impregnation the solids are dried and calcined directly, whereas in the case of the ion-exchange technique the excess liquid is removed from the solids prior to drying and calcination. As shown in U.S. Pat. No. 4,104,320, the ion exchange process may result in residual acidity when, during the subsequent reduction of the platinum cations which are now near atomic dispersion inside the zeolite channels, hydrogen ions are formed in order to maintain charge neutrality of the zeolite structure. The acidity occurs because a large fraction of the non-framework cations that were displaced by platinum cations during loading is removed in the discarded excess liquid prior to drying and calcination. Subsequently, when the platinum is reduced using hydrogen-containing reducing agents these cations are no longer available to displace protons from these sites. The formation of acid sites is not a problem with the impregnation technique since the displaced ion will remain on the carrier so that when the platinum is subsequently reduced the original displaced ion can replace the proton on these sites. Impregnation would appear to be the desired technique for loading a non-acidic carrier such as the Zeolite-L which needs to remain non-acidic. However, in order to achieve uniform contacting of large quantities of the carrier with the cationic platinum solution during the use of the impregnation technique it becomes necessary to expose the Zeolite-L pellets to inordinant mechanical stress as the result of mixing and tumbling, and as a result, excessive attrition is caused in this type of operation.
U.S. Pat. No. 4,416,806 also is said to disclose the depositing of platinum on a Zeolite L carrier by impregnation and exchange of ions. Also disclosed is that the carrier is immersed in a solution containing platinum for a period of time, washed and dried, and that ion exchange and impregnation may be carried out in the presence of an excess of salt of the cation of the zeolite; for instance, potassium chloride for the KL Zeolite. In U.S. Pat. No. 3,226,339 an aluminosilicate zeolite is contacted with a solution of an ionizable platinum compound and an ionizable non-platinum metal salt for a sufficient period of time said to effect uniform distribution of the platinum ion on the zeolite. While both of these patents discuss the presence of an excess of a metal salt, there is no disclosure of the particular process which is necessary to prevent acid site generation upon the drying, calcination and reduction of the zeolite carrier while avoiding an excess of metal ions in the form of a salt which could block the passage of hydrocarbons through the pores of the zeolite carrier.
In U.S. Pat. No. 3,775,502 Zeolite X is mixed in an ion exchange procedure with a platinum salt and a sodium salt for several hours. Thereafter, the catalyst is washed thoroughly to remove the salt residue and then dried. Excessive water washing at this stage can cause other undesirable reactions, such as the loss of platinum from the carrier and incorporation of acidity into the carrier. Upon reduction the catalyst is given a final treatment of aqeuous sodium bicarbonate salt to convert the H.sup.+ zeolite sites which have been created (also see U.S. Pat. No. 3,953,365).